Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 9475                                       Neustar
Category: Standards Track                                       C. Wendt
ISSN: 2070-1721                                                    Somos
                                                          December 2023


   Messaging Use Cases and Extensions for Secure Telephone Identity
                           Revisited (STIR)

Abstract

  Secure Telephone Identity Revisited (STIR) provides a means of
  attesting the identity of a telephone caller via a signed token in
  order to prevent impersonation of a calling party number, which is a
  key enabler for illegal robocalling.  Similar impersonation is
  sometimes leveraged by bad actors in the text and multimedia
  messaging space.  This document explores the applicability of STIR's
  Personal Assertion Token (PASSporT) and certificate issuance
  framework to text and multimedia messaging use cases, including
  support for both messages carried as a payload in SIP requests and
  messages sent in sessions negotiated by SIP.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9475.

Copyright Notice

  Copyright (c) 2023 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (https://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Terminology
  3.  Applicability to Messaging Systems
    3.1.  Message Sessions
    3.2.  PASSporTs and Individual Messages
      3.2.1.  PASSporT Conveyance with Messaging
  4.  Certificates and Messaging
  5.  IANA Considerations
    5.1.  JSON Web Token Claims Registration
    5.2.  PASSporT Type Registration
  6.  Privacy Considerations
  7.  Security Considerations
  8.  References
    8.1.  Normative References
    8.2.  Informative References
  Acknowledgments
  Authors' Addresses

1.  Introduction

  The STIR problem statement [RFC7340] describes widespread problems
  enabled by impersonation in the telephone network, including illegal
  robocalling, voicemail hacking, and swatting.  As telephone services
  are increasingly migrating onto the Internet and using Voice over IP
  (VoIP) protocols such as SIP [RFC3261], it is necessary for these
  protocols to support stronger identity mechanisms to prevent
  impersonation.  [RFC8224] defines a SIP Identity header capable of
  carrying PASSporT [RFC8225] objects in SIP as a means to
  cryptographically attest that the originator of a telephone call is
  authorized to use the calling party number (or, for SIP cases, SIP
  URI) associated with the originator of the call.

  However, the problem of bulk, unsolicited commercial communications
  is not limited to telephone calls.  Spammers and fraudsters are
  increasingly turning to messaging applications to deliver undesired
  content to consumers.  In some respects, mitigating these unwanted
  messages resembles the email spam problem; for example, textual
  analysis of the message contents can be used to fingerprint content
  that is generated by spammers.  However, encrypted messaging is
  becoming more common, and analysis of message contents may no longer
  be a reliable way to mitigate messaging spam in the future.  As STIR
  sees further deployment in the telephone network, the governance
  structures put in place for securing telephone-network resources with
  STIR could be repurposed to help secure the messaging ecosystem.

  One of the more sensitive applications for message security is
  emergency services.  As next-generation emergency services
  increasingly incorporate messaging as a mode of communication with
  public safety personnel (see [RFC8876]), providing an identity
  assurance could help to mitigate denial-of-service attacks and
  ultimately help to identify the source of emergency communications in
  general (including swatting attacks, see [RFC7340]).

  Therefore, this specification explores how the PASSporT mechanism
  defined for STIR could be applied in providing protection for textual
  and multimedia messaging, but it focuses particularly on those
  messages that use telephone numbers as the identity of the sender.
  Moreover, it considers the reuse of existing STIR certificates, which
  are beginning to see widespread deployment for signing PASSporTs that
  protect messages.  For that purpose, it defines a new PASSporT type
  and an element that protects message integrity.  It contains a
  mixture of normative and informative guidance that specifies new
  claims for use in PASSporTs as well as an overview of how STIR might
  be applied to messaging in various environments.

2.  Terminology

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

3.  Applicability to Messaging Systems

  At a high level, PASSporT [RFC8225] claims provide similar value to
  number-based messaging as they do to telephone calls.  A signature
  over the calling and called party numbers, along with a timestamp,
  could already help to prevent impersonation in the mobile-messaging
  ecosystem.

  When it comes to protecting message contents, broadly, there are a
  few ways that the PASSporT mechanism of STIR could apply to
  messaging:

  1.  a PASSporT could be used to securely negotiate a session over
      which messages will be exchanged (see Section 3.1), and

  2.  in sessionless scenarios, a PASSporT could be generated on a per-
      message basis with its own built-in message security (see
      Section 3.2).

3.1.  Message Sessions

  In the first case, SIP negotiates a session in which the media will
  be text messages or MIME content, as, for example, with the Message
  Session Relay Protocol (MSRP) [RFC4975].  This usage of STIR would
  deviate little from [RFC8224].  An INVITE request sent with an
  Identity header containing a PASSporT with the proper calling and
  called party numbers would then negotiate an MSRP session the same
  way that an INVITE for a telephone call would negotiate an audio
  session.  This could be applicable to MSRP sessions negotiated for
  Rich Communication Suite (RCS) [RCC.07].  Note that, if TLS is used
  to secure MSRP (per RCS [RCC.15]), fingerprints of those TLS keys
  could be secured via the "mky" claim of PASSporT using the framework
  described in [RFC8862].  Similar practices would apply to sessions
  that negotiate real-time text over RTP ([RFC4103], [RFC5194]); any
  that can operate over DTLS/SRTP (Secure Real-time Transport Protocol)
  should work with the "mky" PASSporT claim.  For the most basic use
  cases, STIR for messaging should not require any further protocol
  enhancements.

  Current usage of [RFC8224] Identity is largely confined to INVITE
  requests that initiate telephone calls.  RCS-style applications would
  require PASSporTs for all conversation participants, which could
  become complex in multiparty conversations.  Any solution in this
  space would likely require the implementation of STIR-connected
  identity [CONNECT-ID-STIR], but the specification of PASSporT-signed
  session conferencing is outside the scope of this document.

  Also note that the assurance offered by [RFC8862] is "end-to-end" in
  the sense that it offers assurance between an authentication service
  and verification service.  If those are not implemented by the
  endpoints themselves, there are still potential opportunities for
  tampering before messages are signed and after they are verified.
  However, for the most part, STIR does not intend to protect against
  machine-in-the-middle attacks so much as spoofed origination; so the
  protection offered may be sufficient to mitigate nuisance messaging.

3.2.  PASSporTs and Individual Messages

  In the second case described in Section 3, SIP also has a method for
  sending messages in the body of a SIP request: the MESSAGE method
  [RFC3428].  For example, MESSAGE is used in some North American
  emergency services use cases.  The interaction of STIR with MESSAGE
  is not as straightforward as the potential use case with MSRP.  An
  Identity header could be added to any SIP MESSAGE request, but
  without some extension to the PASSporT claims, the PASSporT would
  offer no protection to the message content; it would potentially be
  reusable for cut-and-paste attacks where the Identity header field
  from a legitimate request for one user is reused in a request for a
  different user.  As the bodies of SIP requests are MIME encoded, S/
  MIME [RFC8591] has been proposed as a means of providing integrity
  for MESSAGE (and some MSRP cases as well).  The use of Common
  Presence and Instant Messaging (CPIM) [RFC3862] as a MIME body allows
  the integrity of messages to withstand interworking with protocols
  that are not SIP.  The interaction of STIR certificates with S/MIME
  (see [RFC8226]) for messaging applications would require further
  specification; additionally, PASSporT can provide its own integrity
  check for message contents through a new claim defined to provide a
  hash over message contents.

  In order to differentiate a PASSporT for an individual message from a
  PASSporT used to secure a telephone call or message stream, this
  document defines a new "msg" PASSporT type. "msg" PASSporTs may carry
  a new optional JSON Web Token (JWT) [RFC7519] claim "msgi", which
  provides a digest over a MIME body that contains a text or multimedia
  message.  Authentication services MUST NOT include "msgi" elements in
  PASSporT types other than "msg", but "msgi" is OPTIONAL in "msg"
  PASSporTs, as integrity for messages may be provided by some other
  service (e.g.  [RFC8591]).  Verification services MUST ignore the
  presence of "msgi" in non-"msg" PASSporT types.

  The claim value of the "msgi" claim key is a string that defines the
  crypto algorithm used to generate the digest concatenated by a hyphen
  with a digest string.  Implementations MUST support the hash
  algorithms SHA-256, SHA-384, and SHA-512.  These hash algorithms are
  identified by "sha256", "sha384", and "sha512", respectively.  SHA-
  256, SHA-384, and SHA-512 are part of the SHA-2 set of cryptographic
  hash functions [RFC6234] defined by the US National Institute of
  Standards and Technology (NIST).  [SHA2] implementations MAY support
  additional recommended hash algorithms in the "COSE Algorithms"
  registry (https://www.iana.org/assignments/cose); that is, the hash
  algorithm has "Yes" in the "Recommended" column of the IANA registry.
  Hash algorithm identifiers MUST use only lowercase letters, and they
  MUST NOT contain hyphen characters.  The character following the
  algorithm string MUST be a hyphen character ("-" or ASCII character
  45).

  The subsequent characters in the claim value are the base64-encoded
  [RFC4648] digest of a canonicalized and concatenated string or
  binary-data-based MIME body of the message.  An "msgi" message digest
  is computed over the entirety of the MIME body (be it carried via SIP
  or not); per [RFC3428], this may be any sort of MIME body, including
  a multipart body in some cases, especially when multimedia content is
  involved.  Those MIME bodies may or may not contain encrypted content
  or as the sender desires.  The digest becomes the value of the JWT
  "msgi" claim, as per this example:

  "msgi" :
  "sha256-H8BRh8j48O9oYatfu5AZzq6A9RINQZngK7T62em8MUt1FLm52t+eX6xO"

  Per [RFC8224], this specification leaves it to local policy to
  determine how messages are handled after verification succeeds or
  fails.  Broadly, if a SIP-based verification service wants to
  communicate back to the sender that the "msgi" hash does not
  correspond to the received message, using a SIP 438 response code
  would be most appropriate.

  Note that, in some CPIM environments, intermediaries may add or
  consume CPIM headers used for metadata in messages.  MIME-layer
  integrity protection of "msgi" would be broken by a modification
  along these lines.  Any such environment would require a profile of
  this specification that reduces the scope of protection only to the
  CPIM payload, as discussed in Section 9.1 of [RFC8591].

  Finally, note that messages may be subject to store-and-forward
  treatment that differs from delivery expectations of SIP
  transactions.  In such cases, the expiry freshness window recommended
  by [RFC8224] may be too strict, as routine behavior might dictate the
  delivery of a MESSAGE minutes or hours after it was sent.  The
  potential for replay attacks can, however, be largely mitigated by
  the timestamp in PASSporTs; duplicate messages are easily detected,
  and the timestamp can be used to order messages displayed in the user
  inbox in a way that precludes showing stale messages as fresh.
  Relaxing the expiry timer would require support for such features on
  the receiving side of the message.

3.2.1.  PASSporT Conveyance with Messaging

  If the message is being conveyed in SIP, via the MESSAGE method, then
  the PASSporT could be conveyed in an Identity header in that request.
  The authentication and verification service procedures for populating
  that PASSporT would follow the guidance in [RFC8224], with the
  addition of the "msgi" claim defined in Section 3.2.

  In text messaging today, Multimedia Messaging Service (MMS) messages
  are often conveyed with SMTP.  Thus, there is a suite of additional
  email security tools available in this environment for sender
  authentication, such as "Domain-based Message Authentication,
  Reporting, and Conformance (DMARC)" [RFC7489].  The interaction of
  these mechanisms with STIR certificates and/or PASSporTs would
  require further study and is outside the scope of this document.

  For other cases where messages are conveyed by some protocol other
  than SIP, that protocol itself might have some way of conveying
  PASSporTs.  There will surely be cases where legacy transmission of
  messages will not permit an accompanying PASSporT; in such a
  situation, something like out-of-band [RFC8816] conveyance would be
  the only way to deliver the PASSporT.  For example, this may be
  necessary to support cases where legacy Short Message Peer-to-Peer
  [SMPP] systems cannot be upgraded.

  A MESSAGE request can be sent to multiple destinations in order to
  support multiparty messaging.  In those cases, the "dest" claim of
  the PASSporT can accommodate the multiple targets of a MESSAGE
  without the need to generate a PASSporT for each target of the
  message.  However, if the request is forked to multiple targets by an
  intermediary later in the call flow, and the list of targets is not
  available to the authentication service, then that forking
  intermediary would need to use diversion PASSporTs [RFC8946] to sign
  for its target set.

4.  Certificates and Messaging

  "Secure Telephone Identity Credentials: Certificates" [RFC8226]
  defines a way to issue certificates that sign PASSporTs, which attest
  through their TNAuthList a Service Provider Code (SPC) and/or a set
  of one or more telephone numbers.  This specification proposes that
  the semantics of these certificates should suffice for signing for
  messages from a telephone number without further modification.

  Note that the certificate referenced by the "x5u" of a PASSporT can
  change over time due to certificate expiry/rollover; in particular,
  the use of short-lived certificates can entail rollover on a daily
  basis or even more frequently.  Thus, any store-and-forward messaging
  system relying on PASSporTs must take into account the possibility
  that the certificate that signed the PASSporT, though valid at the
  time the PASSporT was generated, could expire before a user reads the
  message.  This might require:

  *  storing some indicator of the validity of the signature and
     certificate at the time the message was received, or

  *  securely storing the certificate along with the PASSporT

  so that the "iat" claim can be compared with the expiry freshness
  window of the certificate prior to validation.

  As the "orig" and "dest" claims of PASSporTs may contain URIs without
  telephone numbers, the STIR for messaging mechanism contained in this
  specification is not inherently restricted to the use of telephone
  numbers.  This specification offers no guidance on appropriate
  certification authorities for designing "orig" values that do not
  contain telephone numbers.

5.  IANA Considerations

5.1.  JSON Web Token Claims Registration

  IANA has added one new claim to the "JSON Web Token Claims" registry
  that was defined in [RFC7519].

  Claim Name:  msgi

  Claim Description:  Message Integrity Information

  Change Controller:  IETF

  Specification Document(s):  RFC 9475

5.2.  PASSporT Type Registration

  This specification defines one new PASSporT type for the "Personal
  Assertion Token (PASSporT) Extensions" registry defined in [RFC8225].

  ppt value:  msg

  Reference:  Section 3.2 of RFC 9475

6.  Privacy Considerations

  Signing messages or message sessions with STIR has little direct
  bearing on the privacy of messaging for SIP as described in [RFC3428]
  or [RFC4975].  An authentication service signing a MESSAGE method may
  compute the "msgi" hash over the message contents; if the message is
  in cleartext, that will reveal its contents to the authentication
  service, which might not otherwise be in the call path.

  The implications for anonymity of STIR are discussed in [RFC8224],
  and those considerations would apply equally here for anonymous
  messaging.  Creating an "msg" PASSporT does not add any additional
  privacy protections to the original message content.

7.  Security Considerations

  This specification inherits the security considerations of [RFC8224].
  The carriage of messages within SIP per Section 3.2 has a number of
  security and privacy implications as documented in [RFC3428], which
  are expanded in [RFC8591]; these considerations apply here as well.
  The guidance about store-and-forward messaging and replay protection
  in Section 3.2 should also be recognized by implementers.

  Note that a variety of protocols that are not SIP, both those
  integrated into the telephone network and those based on over-the-top
  applications, are responsible for most of the messaging that is sent
  to and from telephone numbers today.  Introducing this capability for
  SIP-based messaging will help to mitigate spoofing and nuisance
  messaging for SIP-based platforms only.

8.  References

8.1.  Normative References

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E.
             Schooler, "SIP: Session Initiation Protocol", RFC 3261,
             DOI 10.17487/RFC3261, June 2002,
             <https://www.rfc-editor.org/info/rfc3261>.

  [RFC3428]  Campbell, B., Ed., Rosenberg, J., Schulzrinne, H.,
             Huitema, C., and D. Gurle, "Session Initiation Protocol
             (SIP) Extension for Instant Messaging", RFC 3428,
             DOI 10.17487/RFC3428, December 2002,
             <https://www.rfc-editor.org/info/rfc3428>.

  [RFC3862]  Klyne, G. and D. Atkins, "Common Presence and Instant
             Messaging (CPIM): Message Format", RFC 3862,
             DOI 10.17487/RFC3862, August 2004,
             <https://www.rfc-editor.org/info/rfc3862>.

  [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
             Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
             <https://www.rfc-editor.org/info/rfc4648>.

  [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
             (SHA and SHA-based HMAC and HKDF)", RFC 6234,
             DOI 10.17487/RFC6234, May 2011,
             <https://www.rfc-editor.org/info/rfc6234>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
             "Authenticated Identity Management in the Session
             Initiation Protocol (SIP)", RFC 8224,
             DOI 10.17487/RFC8224, February 2018,
             <https://www.rfc-editor.org/info/rfc8224>.

  [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
             Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
             <https://www.rfc-editor.org/info/rfc8225>.

  [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
             Credentials: Certificates", RFC 8226,
             DOI 10.17487/RFC8226, February 2018,
             <https://www.rfc-editor.org/info/rfc8226>.

8.2.  Informative References

  [CONNECT-ID-STIR]
             Peterson, J. and C. Wendt, "Connected Identity for STIR",
             Work in Progress, Internet-Draft, draft-ietf-stir-rfc4916-
             update-04, 23 October 2023,
             <https://datatracker.ietf.org/doc/html/draft-ietf-stir-
             rfc4916-update-04>.

  [RCC.07]   GSMA, "Rich Communication Suite 8.0 Advanced
             Communications Services and Client Specification", Version
             9.0, May 2018, <https://www.gsma.com/futurenetworks/wp-
             content/uploads/2019/09/RCC.07-v9.0.pdf>.

  [RCC.15]   GSMA, "IMS Device Configuration and Supporting Services",
             Version 7.0, October 2019, <https://www.gsma.com/newsroom/
             wp-content/uploads//RCC.15-v7.0.pdf>.

  [RFC4103]  Hellstrom, G. and P. Jones, "RTP Payload for Text
             Conversation", RFC 4103, DOI 10.17487/RFC4103, June 2005,
             <https://www.rfc-editor.org/info/rfc4103>.

  [RFC4975]  Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
             "The Message Session Relay Protocol (MSRP)", RFC 4975,
             DOI 10.17487/RFC4975, September 2007,
             <https://www.rfc-editor.org/info/rfc4975>.

  [RFC5194]  van Wijk, A., Ed. and G. Gybels, Ed., "Framework for Real-
             Time Text over IP Using the Session Initiation Protocol
             (SIP)", RFC 5194, DOI 10.17487/RFC5194, June 2008,
             <https://www.rfc-editor.org/info/rfc5194>.

  [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
             Telephone Identity Problem Statement and Requirements",
             RFC 7340, DOI 10.17487/RFC7340, September 2014,
             <https://www.rfc-editor.org/info/rfc7340>.

  [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
             Message Authentication, Reporting, and Conformance
             (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
             <https://www.rfc-editor.org/info/rfc7489>.

  [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
             (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
             <https://www.rfc-editor.org/info/rfc7519>.

  [RFC8591]  Campbell, B. and R. Housley, "SIP-Based Messaging with S/
             MIME", RFC 8591, DOI 10.17487/RFC8591, April 2019,
             <https://www.rfc-editor.org/info/rfc8591>.

  [RFC8816]  Rescorla, E. and J. Peterson, "Secure Telephone Identity
             Revisited (STIR) Out-of-Band Architecture and Use Cases",
             RFC 8816, DOI 10.17487/RFC8816, February 2021,
             <https://www.rfc-editor.org/info/rfc8816>.

  [RFC8862]  Peterson, J., Barnes, R., and R. Housley, "Best Practices
             for Securing RTP Media Signaled with SIP", BCP 228,
             RFC 8862, DOI 10.17487/RFC8862, January 2021,
             <https://www.rfc-editor.org/info/rfc8862>.

  [RFC8876]  Rosen, B., Schulzrinne, H., Tschofenig, H., and R.
             Gellens, "Non-interactive Emergency Calls", RFC 8876,
             DOI 10.17487/RFC8876, September 2020,
             <https://www.rfc-editor.org/info/rfc8876>.

  [RFC8946]  Peterson, J., "Personal Assertion Token (PASSporT)
             Extension for Diverted Calls", RFC 8946,
             DOI 10.17487/RFC8946, February 2021,
             <https://www.rfc-editor.org/info/rfc8946>.

  [SHA2]     National Institute of Standards and Technology (NIST),
             "Secure Hash Standard (SHS)", FIPS PUB 180-3, 2008,
             <http://csrc.nist.gov/publications/fips/fips180-3/
             fips180-3_final.pdf>.

  [SMPP]     SMS Forum, "Short Message Peer-to-Peer Protocol
             Specification", Version 5.0, February 2003,
             <https://smpp.org/SMPP_v5.pdf>.

Acknowledgments

  We would like to thank Christer Holmberg, Brian Rosen, Ben Campbell,
  Russ Housley, and Alex Bobotek for their contributions to this
  specification.

Authors' Addresses

  Jon Peterson
  Neustar, Inc.
  Email: [email protected]


  Chris Wendt
  Somos
  Email: [email protected]

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